Biologically active materials exposed to high energy electrons are damaged. If the irradiation is carried out at very low temperatures, the rates of loss of activity or structure with radiation dose are directly proportional to the mass of the molecules which are required in the assay. the presence of other molecules has no effect on the measurement of mass; purified samples are not required. Target theory of the inactivation of biological activity by ionizing radiation was extended to several complex biochemical systems. Target size analysis has been applied to several major problems including the molecular forms of extracellular phospholipases A2 and various Golgi enzymes.